The invention relates to the removal of volatile organic compounds from latices/dispersions of synthetic polymers.
The terms latices/dispersions as used herein are also meant to cover emulsions and suspensions.
Latices/dispersions of synthetic polymers are usually prepared by polymerizing unsaturated monomeric materials in a liquid medium, usually water, under the influence of free radicals and at the end of the polymerization stage the latices/dispersions typically contain volatile organic compounds which result from incomplete conversion of monomers, impurities of raw materials and undesirable by-products formed during the polymerization reaction. The presence of these volatile organic compounds (VOC) i.e. having a boiling point below 213.5xc2x0 C. (boiling point of 2-ethylhexyl acrylate) at atmospheric pressure (101.325 kPa) in latices/dispersions of synthetic polymers is undesirable for various reasons and therefore various processes have been developed to remove these compounds. There is still a need for an efficient process to minimize the volatile organic compound (VOC) content of latices/dispersions of synthetic polymers. Especially for such a process which is friendly from an environmental point of view by increasing the yield of polymeric materials to a maximum and decreasing the amount of residual monomer and other volatiles in combination with low energy consumption.
The prior art discloses several methods.
A. Venting the reaction vessel in which the latex or dispersion has been manufactured. This process helps especially in cases where very volatile monomeric materials like e.g. ethylene are polymerized. Cf e.g. U.S. Pat. No. 3,534,009, in the Example.
B. Post-polymerization i.e. adding fresh radical generating materials after the main polymerization to polymerize residual monomeric material further. Cf e.g. U.S. Pat. No. 3,534,009 and EP Appl. 158,523.
C. Sparging air and/or water vapour through the latex/dispersion often called steam stripping is also a well known process, but when carried out in a conventional reactor it is rather time consuming and lowers the throughput of the equipment. Attempts have been made to make steam stripping more efficient e.g. by carrying out the process in special equipment like a degassing column and passing through gas and/or water vapour upwards and the latex/dispersion downwards in countercurrent. Cf e.g. U.S. Pat. No. 4,062,662, EP Appl. 584,458 and WO 97/45,184.
D. Chemical methods like hydrolysing and/or oxidising the residual monomeric materials. Cf e.g. EP Appl. 505,959.
As it is desirable to decrease the VOC level of several percents immediately after polymerization to a few or a fraction of a ppm (part per million) combinations of two or more of the above methods have been developed and disclosed.
One such combination is disclosed in EP Appl. 563,726 and combines chemical oxidation with subsequently distilling off of the residual vinylester and acetaldehyde formed. Another combination is disclosed in EP Appl. 650,977 and involves post polymerization treatment with a free radical generator under polymerization conditions followed by steam stripping under vacuum and results in polymer latices/dispersions containing 5 to 500 ppm of residual monomer and other volatiles. WO 99/14,248 (BASF) discloses the reduction of residual monomer content of e.g. latex by post-polymerization in a reactor having a mixing time which is as short as possible by dosing slowly at least one the components of the redoxinitiator in such a manner that the dosing time is about 10 to 250 times the mixing time of the liquid system in the reactor. It is preferred that the dosing of the oxidizing- and reducing components of the initiator system to the liquid in the reactor is carried out at separate points of addition (xe2x80x9crxc3xa4umlich getrennte Zugabexe2x80x9d) such as the reactor lid, reactor bottom or reactor side wall and that the components are dosed simultaneously or one after the other. There are 12 Examples which specify that the oxidizing agent (tert.-butylhydroperoxide) was dosed first and the reducing agent (e.g. sodium disulphite) was dosed subsequently. There is, however, one exception viz. Example 5b which specifies the simultaneous addition of both components. The tenor is that the sequence order of dosing oxidizing agent and reducing agent does not matter, although the examples seem to indicate that there is some preference for first dosing the oxidizing agent and then the reducing agent.
The present invention relates to an improvement of the process of EP appl. 650,977 and secures one or more of the following advantages: a lower VOC contents in the endproduct and/or a higher throughput than with the current processes and/or a lower energy consumption and/or a better endproduct and/or a higher economy of chemicals used.
In a first embodiment the invention therefore provides a process for treating an aqueous polymer latex/dispersion prepared by polymerizing one or more monomeric materials for at least 98%, comprising the steps of:
A. adding to the polymer latex/dispersion in a sufficient amount of at least one reducing agent in one or more portions followed by adding at least one free radical generator in such a way that at least the bulk of the reducing agent has been added before addition of free radical generator is started and maintaining the mixture at a suitable temperature for a period of time which is sufficient to reduce the VOC-level of the latex/dispersion to decrease to below 1,100 ppm, preferably to below 500 ppm after which
B. water vapour and/or gas is/are fed into the latex/dispersion whilst the temperature of the latex/dispersion is maintained at a suitable temperature for a period of time which is sufficient to decrease the VOC-level to below 200 ppm, preferably to below 50 ppm, more preferably to below 15 ppm. Typically to 0.5-10 ppm.
In step A, only monomeric VOC""s will be reduced by polymerization and formation of polymer, while in step B, all VOC""s that are present will be removed, including non-polymerisable VOC""s that were present in the monomers or other raw materials, or that were produced during the polymerisation. Therefore the combination of steps A and B is particularly advantageous. This combination has both environmental and economical advantages because step A improves the yield of polymer and reduces the amount of volatile material to be removed in step B by a relatively short thermal treatment under mild conditions resulting in a better endproduct with less volatiles and less polymer degradation.
The aqueous polymer latex/dispersion treated according to the present invention is prepared by polymerizing a mixture of one or more unsaturated monomeric materials in an aqueous medium under the influence of a free radical initiator. Emulsion and suspension polymerization techniques are well-known in the art and are e.g. disclosed in xe2x80x9cEmulsion polymerizationxe2x80x9d, D. C. Blackley, Applied Science Publishers, 1974 and xe2x80x9cTextbook of Polymer Sciencexe2x80x9d, F. W. Billmeyer. Wiley, 1975, Chapter 12, which are incorporated by reference. In these polymerizations preferably a suitable free radical generator system is used. For example, free radical generating compounds, ultraviolet light or radiation can be used. The choice of the free radical generating chemical compound to be used depends on the desired polymerization rate and final polymer properties. Some representative examples of free radical initiators which are commonly used include the various ammonium and alkali metal salts of persulphuric or peracetic acid, such as e.g. ammonium persulphate, sodium persulphate, potassium persulphate, sodium peracetate, peroxides such as e.g. hydrogen peroxide, benzoyl peroxide, tertiarybutyl hydroperoxide, tertiaryamyl hydroperoxide, cumene hydroperoxide, paramenthane hydroperoxide, diisopropylbenzene hydroperoxide or percarbonates such as e.g. bis(4-tertiarybutylcyclohexyl)peroxydicarbonate, tertiarybutyl peroctoate and tertiarybutyl perpivalate and azo compounds such as e.g. azobisisobutyronitrile or 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride. These can be used alone, undergoing homolytic cleavage upon heating to yield free radicals, or in conjunction with a reducing agent in a redox couple. Suitable reducing agents include oxidisable sulphur compounds such as alkali metal sulphites, hydrogen sulphites, metabisulphites, dithionites, formaldehyde sulphoxylates or thiosulphates, particularly the sodium salts, ascorbic acid, erythrobic acid and tartaric acid. When a redox couple is used, a suitable compound e.g. salt of a metal which can exist in more than one oxidation state is often included as a co-catalyst or activator. Various transition metal salts can be used, such as e.g. salts of iron, copper or cobalt, but the use of an iron salt, (e.g. ferric chloride or ferrous ammonium sulphate), is preferred. The amount of transition metal is typically up to 30 ppm based on the weight of monomer, preferably up to 20 ppm in either step.
The unsaturated monomeric materials can be diverse and depend on the type of properties desired in the eventual polymeric materials. Frequently used as the main comonomers are vinyl esters of aliphatic carboxylic acids containing 1-20 carbon atoms such as e.g. vinyl acetate, vinyl propionate or vinyl versatate, ethylene, C1-C18 alkyl esters of acrylic acid or methacrylic acid, such as e.g. methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-ethylthexyl acrylate, methyl methacrylate, or n-butyl methacrylate, dialkyl esters and half esters of maleic and fumaric acid containing 1-8 carbon atoms in each alkyl group, such as e.g. dibutyl maleate, vinyl aromatic compounds such as styrene, unsaturated nitriles such as acrylonitrile or methacrylonitrile, less preferably vinyl halide compounds such as vinyl chloride or vinylidene chloride and butadiene.
Optionally other monomers can also be included in lower amounts to impart special properties. These can include unsaturated carboxylic acids, such as e.g. acrylic acid, methacrylic acid, crotonic acid, vinylacetic acid or carboxyethyl acrylate, or dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, or citraconic acid, hydroxyalkyl acrylates or methacrylates with 1-4 carbon atoms in the alkyl(oxy) group such as e.g. hydroxyethyl acrylate, hydroxypropyl acrylate or hydroxyethyl methacrylate, unsaturated amides such as acrylamide or methacrylamide, ionic monomers such as e.g. 2-acrylamido-2-methylpropanesulphonic acid and its salts, sodium vinyl sulphonate or sodium styrene sulphonate. Polyunsaturated monomers may also be included, such as e.g. vinyl crotonate, allyl acrylate or methacrylate, diallyl maleate or fumarate, divinyl adipate, diallyl adipate, diallyl phthalate, ethylene glycol dimethacrylate, butane diol dimethacrylate, methylene bis-acrylamide, triallyl cyanurate and isocyanurate, allyl glycidyl ether or divinyl benzene. Post-polymerisation cross-linking monomers can also be present, such as N-methylol acrylamide, N-methylol methacrylamide, N-methylol allyl carbamate, isobutoxymethyl acrylamide and N-butoxymethyl acrylamide. Preferred are N-methylol acrylamide or a blend of N-methylol acrylamide and acrylamide, available from Cytec Industries Inc, West Paterson, N.J., USA as NMA Special, and silanes such as e.g. vinyl tris(2-methoxyethoxy)silane or vinyl triethoxysilane.
Usually a blend of monomeric materials is used and preferably the blend comprises vinylacetate and a co-monomer. The term monomeric materials also comprises any cross-linking agents present such as e.g. N-methylol acrylamide, N-methylol methacrylamide etc. Preferably the blend of monomeric materials comprise only C, H, O and N atoms, more preferably only C, H and O atoms. In a preferred embodiment of the invention the monomeric materials comprise for at least 50-100%, preferably at least 70% vinylacetate with ethylene and/or acrylate as comonomers or 0-60%, preferably 5-50% styrene with acrylate or methacrylate as comonomer(s) which yield excellent results in the practice of the present invention.
The polymerisation can be carried out using either a batch process, a continuous monomer addition or an incremental monomer addition. The entire amount of the aqueous medium with polymerisation additives can be present in the polymerisation vessel before introduction of the monomers, or alternatively part or all of it can be added continuously or incrementally, either as a separate feed or emulsified with the monomers. A polymer seed can optionally be present, in amounts from 0.1% to 8% by weight on total polymer. The copolymer is generally prepared with an emulsifier and/or a protective colloid which are known in the art. Suitable emulsifiers include anionic, cationic, or nonionic surface active compounds or mixtures thereof. Suitable anionic emulsifiers are, for example, alkyl sulphonates, alkylaryl sulphonates, alkyl sulphates, sulphates of hydroxylalkanols, alkyl and alkylaryl disulphonates, sulphonated fatty acids, sulphates and phosphates of polyethyoxylated alkanols and alkylphenols, as well as esters of sulphosuccinic acid. Suitable cationic emulsifiers are, for example, alkyl quaternary ammonium salts, and alkyl quaternary phosphonium salts. Examples of suitable non-ionic emulsifiers are the addition products of 5 to 50 moles of ethylene oxide adducted to straight-chain and branch-chain alkanols with 6 to 22 carbon atoms, or alkylphenols, or higher fatty acids, or higher fatty acid amides, or primary and secondary higher alkyl amines; as well as block copolymers of propylene oxide with ethylene oxide and mixtures thereof. When combinations of emulsifying agents are used, it is advantageous to use a relatively hydrophobic emulsifying agent in combination with a relatively hydrophillic agent, or to use an anionic emulsifying agent in combination with a nonionic agent. The amount of emulsifying agent is generally from about 0.25 to 10%, preferable from about 0.5 to 6% of the monomers used in the polymerisation. Suitable protective colloids include polyvinyl alcohols, ionically modified starches, water-soluble starches, starch ethers, polyacrylic acid, carboxymethyl cellulose, natural gums such as gum arabic, gelatin, casein, synthetic polymers, and water-soluble cellulose ethers such as hydroxyethyl cellulose. In general, these colloids are used at levels of 0.5 to 10% by weight of the monomers used in the polymerisation.
The free radical emulsion polymerization will typically be conducted at a temperature which is within the range of about 30xc2x0 C. to about 100xc2x0 C. It is generally preferred for the polymerization to be carried out at a temperature which is with the range of 50xc2x0 C. to about 90xc2x0 C. The pressure at which the polymerisation is carried out depends on the nature of the monomers employed. When highly volatile monomer is employed such as e.g. ethylene, superatmospheric pressures, up to e.g. 8000 Kpa, are necessary, otherwise lower pressures or even atmospheric pressures can be employed. The polymerisation is carried out at a pH of from about 1 to about 7, preferably at a pH of from about 3 to about 5. In order to maintain the pH range, it may be useful to work in the presence of customary buffer systems, for example, in the presence of alkali metal carbonates, alkali metal acetates, and alkali metal phosphates. Other ingredients known in the art to be useful for various purposes in emulsion polymerization, such as acids, salts, chain transfer agents, and chelating agents, can be employed.
The polymerisation reaction is generally continued until the residual monomer content is below about 1%.
Step A of the present invention which aims at decreasing the amount of VOC in the polymer latex/dispersion involves the addition of at least one reducing agent in one or more portions to the polymer latex/dispersion. The total amount added lies between 0.02 and 1, preferably between 0.05 and 0.3 wt % calculated on the polymer. Various reducing agents can be used and suitable examples have been identified above. A catalytic amount of transition metal salt is often added with the reducing agent, particularly when the main polymerisation was carried out using a thermal initiating system, but even when the original polymerisation was initiated by a redox couple, it can be advantageous to add an additional quantity of transition metal catalyst.
After the bulk (say at least 60%, rather at least 80%) of the total amount of reducing agent has been added, a roughly equivalent amount of oxidizing agent such as hydrogen peroxide and/or an organic peroxide is added.
This addition is often slowly, continuously or incrementally. The latex/dispersion is kept at a suitable temperature for the rate of polymerisation of the residual monomers to be rapid. In a preferred embodiment of the invention, the temperature during step A is kept between 30 and 100xc2x0 C., preferably between 45 and 85xc2x0 C.
According to another embodiment of the invention the reducing agent is completely added before the addition of the oxidizing agent is started. The reaction temperature and reaction time should be sufficient to decrease the VOC level in step A. to below 1,100 ppm, preferably to below 500 ppm, most preferably below 300 ppm.
This practice of sequentially adding reducing agent and then oxidizing agent has been found beneficial in that lower VOC levels can be reached and/or quicker reached than by simultaneous addition of reducing agent and oxidizing agent or reached using lesser quantities of reducing and oxidising agents. It might be that a complex between the transition metal and the reducing agent also plays a role.
In a preferred embodiment of the present invention the free radical generator or oxidizing agent(s) comprises at least one water-soluble peroxy compound and at least one oil-soluble organic peroxide. Preferably the water-soluble peroxy compound comprises a compound from the group comprising hydrogen peroxide, persulphuric acid and/or its alkali or ammonium salt. More preferably the oil-soluble peroxy compound comprises a compound from the group comprising tertiary-butyl hydroperoxide, cumene hydroperoxide, para-menthane hydroperoxide, diisopropyl-benzene hydroperoxide, tertiary-butyl perpivalate, benzoyl peroxide or tertiary-butyl peroctoate.
In a preferred embodiment of this invention the polymer latex/dispersion comprises a water-soluble transition metal compound. In case no or not enough transition metal ions are present during the post-polymerization step fresh or more transition metal salt should be added to the latex/dispersion.
It is preferred, but not 100% required, that step B. commences only after the oxidizing agent has been completely added and the post-polymerization step has fully taken place. Reasonable results can be obtained after the bulk (say more than 70%) of the oxidising agent has been added. Some overlap between step A and step B. is possible.
Step B involves feeding water vapour and/or gas (inert gas like nitrogen, carbon dioxide or air) into the latex/dispersion whilst the temperature of the latex/dispersion is maintained at a suitable temperature for a period of time which is sufficient to decrease the VOC-level to the detection level of the gas chromatographic method described below 200 ppm, preferably to below 50 ppm. Feeding water vapour and/or gas through the latex/dispersion can be effected by sparging (bubbling through) this gaseous phase through the liquid latex/dispersion which may still be contained in the reactor or in a subsequent tank. In a preferred embodiment of the present invention step B. is carried out at a temperature between 20 and 90xc2x0 C., preferably between 60 and 80xc2x0 C. According to a more preferred embodiment of the invention step B. is carried out at a total pressure of water vapour and/or gas of between 2 and 70 Kpa (absolute), preferably between 20 and 50 Kpa, for example at atmospheric pressure (101.325 Kpa). According to a more preferred embodiment of the invention step B. involves feeding air preferably saturated with water vapour into the latex/dispersion. More in particular it is preferred to feed air saturated with water vapour with a temperature between 10 and 90xc2x0 C., preferably between 60 and 80xc2x0 C. into the latex/dispersion. Especially recommended is feeding in air saturated with water vapour having a temperature which is equal to the temperature of the latex/dispersion during step B. This practice prevents loss and accumulation of water in the latex/dispersion and prevents blocking of the equipment.
According to a preferred embodiment of the invention step B. is carried out in a column equipped with internals in which column the latex/dispersion is cascaded downwards in counter-current flow with water vapour and/or air. Internals such as random packing, structured packing and especially trays provide multiple stages of mass transfer and are certainly an advantage. According to more preferred embodiment of the invention the trays of the degassing column are provided with openings having the form of slits. This embodiment is advantageous because it enables the formation of a multitude of small bubbles of gaseous phase passing through the latex/dispersion and ensures a better mass transfer. Preferably the slits have a specific slit surface area of 11 to 25, preferably 12 to 20% calculated on the cross sectional surface area of the column. This ensures better performance of the column. Also the average surface area of the slit is between 80 and 200, preferably 125 to 175 mm2. Slots of these dimensions give good performance and are therefore preferred.
According to a preferred embodiment of the invention step B. is carried out in a column equipped with internals in which column the latex/dispersion is cascaded downwards in counter-current flow with water vapour and/or air. Internals such as random packing, structured packing and especially trays provide multiple stages of mass transfer and are certainly an advantage. According to more preferred embodiment of the invention the trays of the degassing column are provided with openings having the form of slits. This embodiment is advantageous because it enables the formation of a multitude of small bubbles of gaseous phase passing through the latex/dispersion and ensures a better mass transfer. Preferably the slits have a specific slit surface area of 11 to 25, preferably 12 to 20% calculated on the cross sectional surface area of the column. This ensures better performance of the column. Also the average surface area of the slit is between 80 and 200, preferably 125 to 175 mm2. Slots of these dimensions give good performance and are therefore preferred.